Cellular wireless communications systems are known, wherein a geographical area is divided into cells, and each cell includes a base station (BS, BTS) for communicating with subscriber units (SUs) (also called remote terminals, mobile units, mobile stations, subscriber stations, or remote users) within the cell. In such a system, there is a need for broadcasting information from a base station to subscriber units within the cell, for example to page a particular subscriber unit in order to initiate a call to that SU, or to send control information to all subscriber units, for example on how to communicate with the base station, the control information including, for example, base station identification, timing, and synchronization data. Such paging and control information is broadcast on what are called common control channels. Because often there is no prior information regarding the location of the remote user(s) that need to receive the paging or control information, or because such information is intended for several users, it is preferable to transmit such signals omnidirectionally, or near omnidirectionally, where omnidirectional in general means that the radiated power pattern of the base station is independent of azimuth and elevation within the prescribed coverage area of the base station. This invention deals with methods and apparatuses for achieving such omnidirectional transmissions.
Some examples of a cellular system to which the present invention can be applied are analog systems using the AMPS standard, digital systems which use variants of the Personal Handy Phone System (PHS) protocol defined by the Association of Radio Industries and Businesses (ARIB) Preliminary Standard, RCR STD-28 (Version 2) December 1995, and digital systems that use the Global System for Mobile communications (GSM) protocol, including the original version, 1.8 GHz version called DCS-1800, and the North American 1.9 GHz personal communications system (PCS) version called PCS-1900, these three called “variants” of GSM herein. The PHS and GSM standards define two general sets of functional channels (also called logical channels): a control channel (CCH) set and a traffic channel (TCH) set. The TCH set includes bi-directional channels for transmitting user data between the subscriber units and a base station. The CCH set includes a broadcast control channel (BCCH), a paging channel (PCH), and several other control channels not of concern herein. The BCCH is a unidirectional downlink channel for broadcasting control information from the base station to the subscriber units that includes system and channel structure information, and the PCH is a one-way downlink channel that broadcasts information from the base station to a selected set of subscriber units, or to a wide area of multiple subscriber units (the paging area), and typically is used to alert a particular remote station of an incoming call. The present invention is applicable to all downlink broadcasts and transmissions. It is especially applicable for BCCH and PCH that are used by a base station to simultaneously transmit common information to more than one subscriber (i.e., to broadcast). It is also applicable to other situations where it is desired to transmit RF energy omnidirectionally or at least with no nulls anywhere in the intended region.
The use of antenna arrays for the radiation of radio frequency (RF) energy is well established in a variety of radio disciplines. For the purposes of transmitting in the downlink from a base station which includes an antenna array to a remote receiver (the subscriber unit), the signal intended for the SU can be provided as input to each of the radiating elements of the array, differing from element to element only by gain and phase factors, usually resulting, by design, in a directional radiation pattern focused at the subscriber unit. The benefits of this sort of transmission strategy include increased gain over that possible using a single radiating element and reduced interference to other co-channel users in the system as compared to transmission by means of a single radiating element. Using such an antenna array, spatial division multiple access (SDMA) techniques also are possible in which the same “conventional channel” (i.e., the same frequency channel in a frequency division multiple access (FDMA) system, timeslot in a time division multiple access (TDMA) system, code in a code division multiple access (CDMA) system, or timeslot and frequency in a TDMA/FDMA system) may be assigned to more than one subscriber unit.
Any downlink signals sent are received by a subscriber unit, and the received signal at such receiving subscriber unit is processed as is well known in the art.
When a signal is sent from a remote unit to a base station (i.e., communication is in the uplink), the base station typically (and not necessarily) is one that uses a receiving antenna array (usually, and not necessarily the same antenna array as for transmission). The base station signals received at each element of the receiving array are each weighted in amplitude and phase by a receive weight (also called spatial demultiplexing weight), this processing called spatial demultiplexing, all the receive weights determining a complex valued receive weight vector which is dependent on the receive spatial signature of the remote user transmitting to the base station. The receive spatial signature characterizes how the base station array receives signals from a particular subscriber unit in the absence of any interference. In the downlink (communications from the base station unit to a subscriber unit), transmission is achieved by weighting the signal to be transmitted by each array element in amplitude and phase by a set of respective transmit weights (also called spatial multiplexing weights), all the transmit weights for a particular user determining a complex valued transmit weight vector which also is dependent on what is called the “downlink spatial signature” or “transmit spatial signature” of the remote user which characterizes how the remote user receives signals from the base station absence of any interference. When transmitting to several remote users on the same conventional channel, the sum of weighted signals is transmitted by the antenna array. This invention is primarily concerned with downlink communications, although the techniques certainly are applicable also to uplink communications when the subscriber unit also uses an antenna array for transmitting and omnidirectional transmission from such a subscriber unit is desired.
In systems that use antenna arrays, the weighting of the signals either in the uplink from each antenna element in an array of antennas, or in the downlink to each antenna element is called spatial processing herein. Spatial processing is useful even when no more than one subscriber unit is assigned to any conventional channel. Thus, the term SDMA shall be used herein to include both the true spatial multiplexing case of having more than one user per conventional channel, and the use of spatial processing with only one user per conventional channel. The term channel shall refer to a communications link between a base station and a single remote user, so that the term SDMA covers both a single channel per conventional channel, and more than one channel per conventional channel. The multiple channels within a conventional channel are called spatial channels. For a description of SDMA systems, see for example, co-owned U.S. Pat. Nos. 5,515,378 (issued May 7, 1996) and 5,642,353 (issued Jun. 24, 1997) entitled SPATIAL DIVISION MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEMS, Roy, III, et al., inventors, both incorporated herein by reference; co-owned U.S. Pat. No. 5,592,490 (issued Jan. 7, 1997) entitled SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS, Barratt, et al., inventors, incorporated herein by reference; co-owned U.S. patent application Ser. No. 08/735,520 (filed Oct. 10, 1996), entitled SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING, Ottersten, et al., inventors, incorporated herein by reference; and co-owned U.S. patent application Ser. No. 08/729,390 (filed Oct. 11, 1996) entitled METHOD AND APPARATUS FOR DECISION DIRECTED DEMODULATION USING ANTENNA ARRAYS AND SPATIAL PROCESSING, Barratt, et al., inventors, incorporated herein by reference. Systems that use antenna arrays to improve the efficiency of communications and/or to provide SDMA sometimes are called smart antenna systems. The above patents and patent applications are collectively referred to herein as “Our Smart Antenna patents.”
Because broadcasting implies the simultaneous transmission of data over a common channel to a dispersed set of subscriber units, it is desirable to find methods for using the multiple element antenna array and associated transmitter hardware for broadcasting both common downlink channel information and traffic information intended for one or more particular users.
Desirable Characteristics
A successful strategy will have the following characteristics:                given no prior information on the likely location of remote receivers, a remote receiver at any azimuth receiving a signal at least once over a time period at approximately the same level as a user at any other location at any time during the time period. This is called “near omnidirectional” (NOR) broadcasting herein;        low variation in the transmit power of each element in the array so that good advantage is taken of all elements in the array and scaling issues that arise in practice are minimized;        significant pattern gain relative to that achievable with a single element of the array transmitting at comparable power to the individual transmission powers of the array elements in the time period; and        low total radiated energy so that all elements are being used efficiently.        
The property “low relative radiated power” herein means low radiated power per antenna element over a time period relative to the power required to effect a comparable maximum radiation level (comparable in range, azimuth and elevation) using a single antenna element of the same gain (e.g., as measured in dBi) as the individual elements of the antenna array. Since the difference in radiated power may translate to different power amplifier requirements, and very high power amplifiers are relatively expensive, in some situations, even 1 dB may be a significant difference in radiated power. In more general cases, 3 dB will be considered a significant difference in radiated power.
Sectorized systems using antenna arrays are known in the art. In a sectorized system, rather than true omnidirectional broadcasting (360° of azimuth coverage) there is a need in the art for broadcasting efficiently in the intended coverage region (i.e., the sector) of the antenna array and associated electronics. Thus, in this document, the term “omnidirectional” will be taken in the following sense: 1) “omnidirectional” means approximately, nearly omnidirectional (“NOR”); 2) in an unsectorized cellular system, omnidirectional will mean NOR for 360° of azimuth coverage, and 3) in a sectorized system, omnidirectional will mean nearly omnidirectional in the intended sector width (e.g., 120° of azimuth coverage for 120° sectors).
The Prior Art
A common method for so broadcasting data is to use an omnidirectional antenna so that the RF carrier is transmitted more-or -less uniformly in all directions. This omnidirectional radiation pattern appears to be a reasonable choice for mobile cellular systems in which the subscriber units can be arbitrarily positioned within the cell area. In the case of a smart antenna system, one can achieve such an omnidirectional pattern either by using a separate single omnidirectional antenna (such as a vertical dipole) or one of the elements in the antenna array (assumed to have m elements). Unfortunately, this would require increasing the total transmitter power in that antenna element (or separate antenna) compared to the power levels used in ordinary TCH communications when all the antenna elements are operational, to achieve similar range for the traffic and control channels. The option of increasing power may not be allowed by regulation and, even if allowed, may not be a practical choice because, for example, power amplifier costs tend to increase rapidly with power.
The prior art method of transmitting from only a single array element would satisfy the desirable criteria of approximately constant gain as a function of azimuth and other quantities that describe the location of the remote receiver, and of low total radiated energy, but would not give low variation in the transmit power of each element in the array so that good advantage is taken of all elements in the array and scaling issues that arise in practice are minimized, and would not provide significant pattern gain relative to that achievable with a single element of the array transmitting at comparable power to the individual transmission powers of the array elements. In addition, transmitting from only one antenna would not enable simultaneous communications with several users on the same conventional channel.
Alternatively, the antenna array radiation pattern may be controlled through a combination of sending multiple beams and applying pre-processing to any signals prior to beamforming. U.S. Pat. No. 5,649,287, (issued Jul. 15, 1997), entitled ORTHOGONALIZING METHODS FOR ANTENNA PATTERN NULLFILLING, Forssen, et al., inventors, discloses a method for sending information in a cellular communication system comprising at least one base station with an antenna array and a plurality of mobile stations. The common information is pre-processed to create orthogonal signals. The orthogonal signals are then beamformed so that the orthogonal signals are delivered to the different beams in the array antenna. The orthogonal signals are transmitted and then received at one or more mobile stations. The signals are then processed at the mobile station to decipher the common information from the orthogonal signals. The orthogonalizing signals to be transmitted to the mobile stations are formed so as to prevent nulls from occurring in the antenna pattern.
The Forssen et al. method requires pre-processing (orthogonalizing) the control signal to form m orthogonal signals which are then fed to a beamformer. That is, any signal to be broadcast is first transformed to a set of uncorrelated signals, and then each of these signals is sent on a different beam. This requires extra hardware or processing steps. In addition, the particular embodiment described by Forssen et al. requires a high performance equalizer at the subscriber unit to resolve the orthogonalized signals from the other various lobes. It would be desirable to use a system in which any signal to be transmitted is weighted only in phase and amplitude without requiring an additional step (e.g., orthogonalization).
Thus there is a need in the art for methods for omnidirectional downlink transmitting that use the existing communications system apparatus including the existing antenna elements in an antenna array to achieve acceptable omnidirectional performance with low relative radiated power. There also is a need in the art for an apparatus that achieves this.